The invention relates to a communications receiver arrangement for the receipt of digital data, in particular, but not exclusively, digital data communicated over an optical-fibre medium.
Optical transmission systems require a conversion from electrical to optical signals at the transmit end and from optical to electrical signals at the receive end. The receive end is required to carry out a clock-recovery and data-detection process, the latter providing a discrimination between the logic levels involved, i.e. logic xe2x80x9c1xe2x80x9d""s and xe2x80x9c0xe2x80x9d""s in the two-level system normally employed. To achieve this it is necessary for the receiver to generate a threshold level in order to determine whether the input signal is at any one time a xe2x80x9c1xe2x80x9d or a xe2x80x9c0xe2x80x9d. Due to the low levels involved at the receive end and the effects of noise and distortion, it is difficult to decide what the threshold level should be to produce reliable detection.
FIG. 1 illustrates this problem and shows on the lefthand side the logic levels of the transmitted signal, in which logic xe2x80x9c1xe2x80x9d is at +5V and logic xe2x80x9c0xe2x80x9d is at slightly above 0V, i.e. a difference of almost 5V between the two levels. In both cases there will be a small amount of uncertainty in the levels due to noise. Due to non-ideal transmission conditions down an optical link by the time the signal reaches a receiver it might look more like the situation on the righthand side of the drawing, where the difference between the two logic levels is very much less than the original difference at transmission. The difference due to distortion could be as little as 200 mV. There is also a pronounced lack of symmetry in the distortion effect, which is common in these communications systems. If now the receiver has a fixed threshold level of around 2.5V or even higher, due to the large amount of noise present on the received signal and the much reduced difference in logic voltage levels, it is likely that some xe2x80x9c1xe2x80x9d""s will drop below this threshold and be read as xe2x80x9c0xe2x80x9d""s. A fewer number ofxe2x80x9c0xe2x80x9ds will also be misread as xe2x80x9c1xe2x80x9d""s. The result will be errors in data detection, but more importantly there will be inequalities in these errors; in the example given, there will be more false xe2x80x9c0xe2x80x9d readings than false xe2x80x9c1xe2x80x9d readings. It is considered desirable to have a system in which the number of errors for the two levels is approximately equal.
In accordance with a first aspect of the invention, there is provided a communications receiver arrangement for the receipt of digital data, comprising: means for receiving digital data encoded in accordance with multiple encoding levels, means for establishing one or more thresholds for detecting the multiple encoding levels, and means for determining the error rate associated with each individual encoding level, the threshold-establishing means comprising a threshold-optimisation means for optimising the one or more thresholds on the basis of the determined error rates.
The threshold-optimisation means may comprise means for comparing the various error rates and means for changing the one or more thresholds on the basis of the comparison result. The comparing means may form a ratio between the various error rates, in which case the threshold-changing means will vary the one or more thresholds until respective predetermined ratios are established between the error rates; alternatively, the comparing means may form a difference between the various error rates, in which case the threshold-changing means will vary the one or more thresholds until respective predetermined differences are established between the error rates. The predetermined ratios may be approximately unity and the differences approximately zero.
The number of encoding levels may be two.
There may be a single threshold discriminating between the two levels. A hysteresis is preferably provided in the threshold in order to provide stability of control.
Preferably the error rates are provided as a by-product of an error-correction system, e.g. a Forward Error Correction system.
In a second aspect of the invention, a method for equalising the spurious-count rates for each of multiple data-encoding levels in a communications receiver comprises: receiving data encoded in accordance with the multiple encoding levels, determining the spurious-count rates, and optimising, on the basis of the determined spurious-count rates, one or more thresholds for detecting the multiple encoding levels.